Object Oriented Programming
Debapriyo Majumdar
Programming and Data Structure Lab
M Tech CS I – Semester I
Indian Statistical Institute Kolkata
August 7 and 14, 2014
Objects
Real world objects, or even people!
A person travels by metro.
Person
Buy ticket
Get in
Scans the ticket
Metro
Driver
Get out
Travels
Runs, Stops
Scan Machine
 Different objects all performing their duties
 Objects are related to each other too
2
Objects
 State and Behavior
 States of the person
– Name, Profession, Current location, …
 States of the metro
– AC/Non-AC, Color, Location, Number of coaches, …
 Behavior: actions performed by the object
– Person: Travel, Attending a class, Eating
– Metro: Running, Stopping, …
3
Independence and Dependency
Metro
Person
Behavior that matters to the person
Behavior that matters to the metro
Opens the door
Buys the ticket
Transports
Avails all the facilities
Closes the door
Announces present and next station
Provides a system for buying tickets
Provides a system for validating tickets
 Consider: some changes in the door opening technology
 The functionality for the passenger will remain the same
– There will be changes in the internal implementation of the
doors
4
Exercise – The PDS Lab
 Exercise: Consider the scenario of this class – PDS
Lab course
 Identify the objects, their states and behaviors, what
do the objects provide to each other
5
The Java Hello World Program
 Create a directory called “isical” inside dayX/java
 Open a file named Main.java inside isical
package isical;
public class Main {
public static void main(String[] args) {
System.out.println("Hello World!");
}
}
6
Compile and Run!
 Your path: XYZ/java/isical/
 Go to the directory XYZ/java
 Compile:
$ javac isical/Main.java
 A file called Main.class should be created
 Run
$ java isical/Main
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Object oriented programming
The instructor to write down the name of all the
students
Overview:
– A list of students is passed to the instructor
– Every student can tell their names
– Instructor has a method to write down the names
8
The Student Object
File: Student.java
package isical;
public class Student {
private String name;
// The name of the student
private int rollNumber;
// The numeric roll number of the student
}
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The Student Object
File: Student.java
package isical;
public class Student {
private String name;
// The name of the student
private int rollNumber;
// The numeric roll number of the student
Now compile Student.java
/**
* Constructs an instance of a student
* @return
*/
public Student(String name, int rollNumber) {
this.name = name;
this.rollNumber = rollNumber;
}
}
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The Student Object
File: Student.java
What does the Student class do for me?
package isical;
Nothing yet!!
public class Student {
The student needs to tell his/her name
private String name;
// The name of the student
and roll number!!
private int rollNumber;
// The numeric roll number of the student
/**
* Constructs an instance of a student
* @return
*/
public Student(String name, int rollNumber) {
this.name = name;
this.rollNumber = rollNumber;
}
}
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The Student Object
File: Student.java
package isical;
public class Student {
…………
ADD THE FOLLOWING LINES
…………
/* What can the student do? */
public String getName() {
return name; // Returns the name
}
Now (s)he can
public int getRollNumber() {
return rollNumber;
}
}
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The Instructor Object
Filename: ?
package isical;
public class Instructor {
private String name; // The name of the student
private String unit; // Which unit does the instructor belong to?
public Instructor(String name, String unit) {
this.name = name;
this.unit = unit;
}
/* What can the student do? */
public String getName() {
return name; // Returns the name
}
What does (s)he need to do?
Get names from students and
write down
public String getUnit() {
return unit; // Returns the name of the unit
}
}
13
The method to do that work
public void writeNamesOfStudents(Student[] listOfStudents) {
for (int index = 0; index < listOfStudents.length;
{
String nameOfStudent = listOfStudents[index].getName();
System.out.println(nameOfStudent);
}
}
index++)
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The final code
Filename: Main.java
package isical;
public class Main {
public static void main(String[] args) {
// Create an array of Students
Student[] listOfStudents = new Student[10];
// Create the students
for (int studentNum = 0; studentNum < 10; studentNum++) {
Student student = new Student("Student Number"+studentNum, studentNum);
listOfStudents[studentNum] = student;
}
// Create an instructor
Instructor instructor = new Instructor("Instructor1","Some Unit Who cares");
// Now get the instructor to do the work
instructor.writeNamesOfStudents(listOfStudents);
}
}
15
What have we learnt so far?
 The basic object oriented way of thinking
 Java
– Basic file organization, packages
– Class, constructor, methods
– One example
16
Inheritance
More
general:
Superclass
MTech
Student
Living
being
Human
being
Faculty
Bird
JRF
More
specific:
Subclass
 Subclasses can use state and behavior of the superclass
 Java: each class can have one direct superclass, each
superclass can have any number of subclasses
17
Student, Instructor and Person
package isical;
public class Person {
private String name; // Name of the person
private int age; // Age of the person
public Person(String name, int age) {
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public int getAge() {
return age;
}
}
18
Student as a subclass of Person
package isical;
public class Student {
Superclass
Student extends Person
private String name; // The name of the student
private int rollNumber; // The numeric roll number of the student
public Student(String name, int age) {
super(name,age); // Call the constructor of the superclass
}
/* What can the student do? */
public String getName() {
return name; // Returns the name
}
public int getRollNumber() {
return rollNumber;
}
}
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Student as a subclass of Person
package isical;
public class Student extends Person {
Superclass
Student extends Person
private String name; // The name of the student
private int rollNumber; // The numeric roll number of the student
public Student(String name, int age) {
super(name,age); // Call the constructor of the superclass
}
/* What can the student do? */
public String getName() {
return name; // Returns the name
}
public int getRollNumber() {
return rollNumber;
}
}
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Student as a subclass of Person
package isical;
public class Student extends Person {
Do we need it?
private String name; // The name of the student
private int rollNumber; // The numeric roll number of the student
public Student(String name, int age) {
super(name,age); // Call the constructor of the superclass
}
/* What can the student do? */
public String getName() {
return name; // Returns the name
}
Do we need it?
public int getRollNumber() {
return rollNumber;
}
}
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Student as a subclass of Person
package isical;
Inherited from
Person
public class Student extends Person {
private String name; // The name of the student
private int rollNumber; // The numeric roll number of the student
public Student(String name, int age) {
super(name,age); // Call the constructor of the superclass
}
/* What can the student do? */
public String getName() {
return name; // Returns the name
}
public int getRollNumber() {
return rollNumber;
}
Inherited from
Person
What else is inherited?
}
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Student as a subclass of Person
package isical;
public class Student extends Person {
State not in
Person
private int rollNumber; // The numeric roll number of the student
public Student(String name, int age) {
super(name,age); // Call the constructor of the superclass
}
/* What can the student do
(more than what it inherits by being a Person) */
public int getRollNumber() {
return rollNumber;
}
}
Now compile, run and verify that a Student can still tell his/her name!
Similarly, modify the code so that the Instructor also extends Person
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Overriding a super method
 Now consider the following functionality in Person
public class Person {
…
…
private String profession; // Profession of the person
…
…
public void setProfession(String profession) {
this.profession = profession;
}
public String getProfession() {
return profession;
}
}
The Student would automatically inherit the same!
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The Student does it differently!
public class Student extends Person {
…
…
private String institute; // The name of the institute
private String program; // The name of the program
…
…
// Overrides isical.Person.getProfession
public String getProfession() {
return "Student of " + program + " at " + institute;
}
}
25
Abstract class
 Abstract method: too abstract to provide an
implementation
– Name of a living being
– Area of a shape (can be circle, rectangle, … )
 Abstract class: Some conceptual objects with one or
more abstract methods
– Concrete subclasses are derived from abstract class
26
LivingBeing
package isical;
public abstract class LivingBeing {
Define an
abstract class
public abstract String getName();
}
public class Person extends LivingBeing {
……
……
}
Abstract method
Derived class
Must implement
the inherited
abstract method
27
Interface
Methods
Outside
world
Object
Interface
 Methods form the object’s interface with the outside
world
 A group of methods to define a set of common
functionalities  An interface
28
An Interface Movable
package isical;
public interface Movable {
Any movable
object must have
this functionality
public int movePosition(int position);
}
public class Person extends LivingBeing implements Movable {
//... ...
@Override
public int movePosition(int position) {
return position+1;
}
In general a
person moves
one step
}
29
Abstract class vs Interface
 Abstract classes, superclasses and derived classes are
supposed to be of the same type of objects
– Living being  Human, Bird
– House  Bungalow, Hut
 An Interface defines a set of functionalities
– House: class, Air conditioning: interface
– Some houses implement air conditioning
 Proper use is left to the developers 
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Parameterization
 Array of 10 integers : int[10]
 Array of 10 booleans : boolean[10]
What are these?
Of what type?
Parameterized classes
 List of Integers : ArrayList<Integer>
 Collection of Books : HashMap<String,Book>
31
Test.java – check things out
package isical;
import java.util.ArrayList;
public class Test {
public static void main(String[] args) {
ArrayList<String> arrayList = new ArrayList<String>();
arrayList.add("First"); arrayList.add("Second");
arrayList.add("Third"); arrayList.add("Another");
arrayList.add("Yet another");
System.out.println(arrayList.size());
System.out.println(arrayList.get(3));
System.out.println(arrayList.get(8));
arrayList.set(4, "XYZ");
System.out.println(arrayList.get(4));
}
}
32
Also check out HashMap
import java.util.HashMap;
// REST OF THE BODY OF THE CLASS SHOULD BE HERE…
HashMap<Integer,String> map = new HashMap<Integer,String>();
map.put(1, "Prateek Pandey");
// Put your roll numbers
// Now get the value given the key
System.out.println("Roll number: " + 1 +
", Name: " + map.get(1));
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A Basic Name Directory
package isical;
/**
* This class stores the names of students per batch
*/
public abstract class NameDirectory {
/**
* Adds a student to the specified batch
*
* @param student
* @param batchName
*/
public abstract void addStudent(Student student, String batchName);
/**
* Given a batch name and student name, finds the student in the batch
*
* @param batchName
* @param studentName
* @return the student object if found, null otherwise
*/
public abstract Student getStudent(String batchName, String studentName);
}
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Implementation : DynNameDirectory
package isical;
public class DynNameDirectory extends NameDirectory {
States?
@Override
public void addStudent(Student student, String batchName) {
// TODO: Implement it
}
@Override
public Student getStudent(String batchName, String studentName) {
// TODO: Implement it
return null;
}
}
35
Implementation : DynNameDirectory
package isical;
public class DynNameDirectory extends NameDirectory {
// Internal storing mechanism
private HashMap<String,ArrayList<Student>> directory
= new HashMap<String,ArrayList<Student>>();
@Override
public void addStudent(Student student, String batchName) {
// TODO: Implement it
}
@Override
public Student getStudent(String batchName, String studentName) {
// TODO: Implement it
return null;
}
}
36
Implementation : DynNameDirectory
package isical;
public class DynNameDirectory extends NameDirectory {
// Internal storing mechanism
private HashMap<String,ArrayList<Student>> directory
= new HashMap<String,ArrayList<Student>>();
Key
(name)
String
ArrayList
Hashmap
Hash
function
Key, Value
Student11
Student12
…
…
Key, Value
Student21
Student22
…
…
Key, Value
37
Implementation : addStudent
package isical;
public class DynNameDirectory extends NameDirectory {
private HashMap<String,ArrayList<Student>> directory
= new HashMap<String,ArrayList<Student>>();
@Override
public void addStudent(Student student, String batchName) {
// Check if the batch exists
ArrayList<Student> list = directory.get(batchName);
// If does not exist, should return null
if (list == null) {
list = new ArrayList<Student>();
}
// Now add the student
list.add(student);
// Now put the list back into the hash map
directory.put(batchName, list);
}
}
38
Implementation : getStudent
public class DynNameDirectory extends NameDirectory {
private HashMap<String,ArrayList<Student>> directory
= new HashMap<String,ArrayList<Student>>();
@Override
public Student getStudent(String batchName, String studentName) {
// Get the list corresponding to the batch
ArrayList<Student> listOfStudents = directory.get(batchName);
// If does not exist, then return null anyway
if (listOfStudents == null) return null;
// Otherwise find the student by iterating over the students
for (Student student : listOfStudents) {
String name = student.getName();
if (name.equalsIgnoreCase(studentName)) {
return student;
}
}
// If nobody is found, return null
return null;
}
}
Iterator
39
Collections, Lists
Iterable,
Collection
Abstract
Collection
List
Set
Abstract
List
ArrayList
Vector
Abstract
Set
……
…
HashSet
TreeSet
…
40
Generics
package isical;
/**
* A pair of two elements. First one is of type T1, the second one is of type T2
*
* @param <T1>
* @param <T2>
*/
public class Pair<T1,T2> {
private T1 element1;
private T2 element2;
Write the methods
to set the first and
second elements
public Pair(T1 e1, T2 e2) {
element1 = e1; element2 = e2;
}
public T1 getFirstElement() {
return element1;
}
Is one pair
comparable to
another?
public T2 getSecondElement() {
return element2;
}
}
41
Generics
package isical;
What do we need for comparing?
public class Pair<T1,T2> {
private T1 element1;
private T2 element2;
}
public class Pair<T1 extends Comparable<T1>,T2>
public class Pair<T1 extends Comparable<T1>,T2> implements Comparable<Pair<T1,T2>> {
@Override
public int compareTo(Pair<T1, T2> o) {
// Simply compare based on element1, because T1 is comparable
return element1.compareTo(o.getFirstElement());
}
}
42
Advanced in-class Exercise
 Implement your own List, called MyList with basic
dynamic element addition feature
 Should support methods to
–
–
–
–
Check current size (number of elements)
Add an element
Get the element at position i
Set an element at position i
 Generic class definition:
public class MyList<T> {
}
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